Cephalothorax
The cephalothorax (call prosoma in some groups) is an anatomical term used in arachnids and malacostracan crustaceans for the first (anterior) major body section. The remainder of the body is the abdomen (opisthosoma), which may also bear lateral appendages as well as the tail, if present. The term "prosoma" can also be applied to the head of insects, but as the two are always exactly synonymous in insects (not true for mesosoma vs. thorax or metasoma vs. abdomen), the simpler term - "head" - is used instead.
Chelicerate cephalothorax
In the chelicerates, the cephalothorax does not originate from any fusion of head + thorax, because there is no post-cephalic tagmosis (no thorax) in their immediate ancestors. A more correct usage is to say that the cephalothorax (prosoma) in the Chelicerata is formed by the fusion of the head segments + some anteriormost trunk segments. Abdomen is too much a general term, indicated unrelated structures in a number of taxa. Recommended usage is prosoma versus opisthosoma. The dorsal sclerites of the cephalothorax are typically fused into a shield called carapace, while the ventral ones are much reduced and usually covered by the coxae of pedipalps and legs I-IV.
In general textbooks on arachnids and their relatives 'prosoma' and 'cephalothorax' are often used interchangeably. Horseshoe crabs are quite unusual in that part of the first two opisthosomal segments have actually become incorporated into their prosoma and here the term cephalothorax has been proposed as a specific name for this modified front part of the horseshoe crab body.
Crustacean cephalothorax
It is derived from the fusion of the head (from Greek κεφαλή cephale) and the hi (from Greek θώραξ thorax), and therefore includes all the mouthparts, antennae, and the thoracic appendages, such as the legs of a lobster. In the Malacostraca, the cephalothorax is typically covered by a protective carapace.
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